Ammonium nitrate propellant compositions

ABSTRACT

Novel propellant compositions comprising ammonium nitrate as the primary oxidizer and thermoplastic phenoxy resin as the binder.

United States Patent 1191 Henderson 11 3,720,553 [451March 13, 1973 AMMONIUM NITRATE PROPELLANT COMPOSITIONS [52] U.S. Cl. ..149/19, 149/20, 149/47,

149/60 [51] Int. Cl. ..C06d 5/06 [58] Field of Search ..149/60, 19, 20, 47

[56] References Cited UNITED STATES PATENTS 3,130,096 4/1964 Pruitt et a1 149/19 3 ,171,764 3/1965 Parker et a1. ..149/1 9 I 3,198,677 8/1965 Thomas ..149/19 3,236,704 2/1966 Axelrod et a1 ..149/19 3,301,187 1/1967 Donaldson et al.... ..149/19 X 3,376,175 4/1968 Sheeline ..149/19 Primary Examiner-Carl D. Quarforth Assistant Examiner-E. A. Miller AttorneyArthur G. Gilkes, William T. McClain and Ralph Charles Medhurst [57] ABSTRACT Novel propellant compositions comprising ammonium nitrate as the primary oxidizer and thermoplastic phenoxy resin as the binder.

5 Claims, No Drawings AMMONIUM NITRATE PROPELLANT COMPOSITIONS BACKGROUND OF THE INVENTION This invention relates to a new class of propellant compositions and more particularly to ammonium nitrate compositions having improved ballistic properties, as a result of a novel binder system.

The use of ammonium nitrate-based compositions as a solid propellant is attractive because of the cheapness 1 and availability of ammonium nitrate, because of the relatively low flame temperature of decomposition of ammonium nitrate, and because the excess free oxygen available from the decomposition permits the use of oxidizable material to improve the energy available from the decomposition. However, the physical characteristics of ammonium nitrate and grain material produced therefrom introduce problems with respect to choice of binder material. Solid ammonium nitrate exists in different crystalline forms at different temperatures and the transition from one form to a different form involves a volume change of the ammonium nitrate. Volume changes which occur at about 90F. and also at about F. are particularly destructive to shaped propellant grains. It is, therefore, obvious that ammonium nitrate-based compositions could be seriously afiected by storage at temperatures common to such storage conditions.

One requirement for solid propellants suitable for military use is that the propellant be ballistically stable after long periods of storage at temperatures as high as 160F. and as low as 65F. Another requirement is that the grain composition not shatter or crack after being subjected to alternate high and low temperatures (i.e., cycled from the high temperature immediately to the low temperature at least twice); and that the buming of the composition following such cycling be uniform and not changed materially from the burning characteristics of such grain material which has not been subjected to such cycling. The binder material used with the ammonium nitrate to form physically stable grains must be flexible enough to compensate for changes in volume of the ammonium nitrate as it passes from one temperature to another, in order that such changes produce a minimum amount of voids and cracks in the grain. Production of fissures in the grain either internally or externally of the surface of the grain creates additional burning surface which results in unpredictability of the ballistic performance of the grain. Furthermore, the binder material must be of such na ture to permit grain formation by methods known to the art, i.e., extrusion, molding, etc.

SUMMARY OF THE INVENTION I have now discovered a novel propellant composition comprising ammonium nitrate as the oxidizer and a thermoplastic phenoxy resin as the binder. The oxidizer and binder must be in intimate physical mixture. The present compositions may also contain various other additives such as catalyst for the promotion of combustion, carbon, chemical stabilizer, etc.

The term ammonium nitrate as used in this specification and in the claims is intended to mean either ordinary commercial grade ammonium nitrate such as conventionally grained ammonium nitrate containing a small amount of impurities which may be coated with a small amount of moisture-resisting material such as petrolatum or paraffin, or to mean military grade ammonium nitrate or mixture of minor amounts (usually less than 10 percent) of other organic or inorganic nitrates such as, guanidine nitrate or sodium nitrate or potassium nitrate with the ammonium nitrate. A mixture of finely ground and unground or coarsely ground ammonium nitrate is preferred. The major proportion of the ammonium nitrate should be finely ground in order to reduce the voids to a minimum and hence avoid the use of excess binder material. It is well known in the propellant art that particle size and size distribution within the composition are related to the ballistic properties. The amount of grinding then, will depend upon the desired properties of the propellant.

The binder material is a high molecular weight thermoplastic resin. The preferred resin may be produced by the reaction of para, para'-isopropyl-idenediphenol (bisphenol-A) and epichlorhydrin. These resins may be obtained commercially from the Union Carbide Corporation under the trade mark Bakelite Phenoxy Resin. The Bakelite Phenoxy Resins have been characterized by the following molecular structure:

wherein n is an interger of such magnitude that the molecular weight of the resin is within the range of about 20,000 to about 40,000, and preferably about 30,000, in which case n would be about 100.

The propellant compositions of the present invention have the advantage of excellent ballistic properties, stability, and simplicity of formulation. The binders provide a tough, flexible material with excellent physical properties in the temperature range normally used with ammonium nitrate propellants. The thermoplastic resins have melt-flow temperatures within the range of about C. to about C. The binder imparts the lower brittle point and considerably higher impact strength at low temperatures, and. accordingly provides superior propellant compositions.

For use as a binder, the phenoxy resins are most desirably combined with a plasticizer. The plasticizer, which has been found to effect the ballistics properties of the composition should be chosen to provide the desired ballistics. Furthermore, the material must be compatible with the propellant system. Many plasticizers are known for use with the present binder. Dimethyl phthalate and dibutyl phthalate exhibit effective properties of a plasticizer and greatly enhance the ballistic properties of the propellant system. Dibutyl phthalate is preferred as a result of the high stability and lower pressure exponent of the propellant system using it. The amount of the plasticizer should be chosen in accordance with the desired ballistics. The ratio of phenoxy resin to plasticizer may be in the range of 1:1 to 10:1 by weight. The ratio will depend upon the particular plasticizer used, and the desired ballistics, and physical properties.

It is desirable that catalyst be present in the composition and any catalyst known to be effective for the combustion of ammonium nitrate grains containing oxidizable thermoplastic binder materials may be used. These include certain iron compounds broadly designated as prussian blues, sodium barbiturate, etc.

It has also been found that the addition of a stabilizer greatly increases the usefulness of the composition. Therefore, when stability under extreme conditions for long periods of time is necessary, a stabilizer component is quite helpful in this respect. However, the present propellant system is entirely satisfactory without stabilizer. A very effective stabilizer in the propellant compositions of the present invention comprises tetranitrocarbazole. The tetranitrocarbazole component, which may be used in conjunction with known stabilizers for ammonium nitrate in thermoplastic propellant compositions may be present in amounts of from 1 to 15 weight percent, based on the weight of the entire composition, and preferably amounts from 1 to 10 percent.

It may also be advantageous to use a carbon component in the composition. The finely divided carbon may be added in amounts up to 10 percent by weight of the composition, preferably in amounts of l to 5 percent by weight for the purpose of improving ignition and the burning rate of the composition. Highly adsorptive activated carbons such as Norite and Nuchar well known in the art as activated carbon of vegetable origin, make up one class of effective burning rate components. A second general class of carbon useful for increasing the burning rate of compositions are the carbon blacks, roughly classified as the channel blacks and the furnace combustion blacks. The carbon blacks are characterized by low ash content, that is, less than about 0.5 percent, usually less than about 0.l5 percent, and by having extremely small particle size, that is, 50 to 5,000 A and contain adsorbed hydrogen and oxygen. Bead type carbon blacks, such as Micronex Beads and Statex Beads, are also suitable. A third type of carbon which may be used is finely ground petroleum coke, particularly petroleum coke obtained as residue in the pipe-stilling of mid-continent heavy residuums.

The oxidizer and binder should be present in the composition in essentially stoichiometric amounts, in order to best utilize the excess oxygen obtained from the oxidizer. Therefore, a major amount of oxidizer and a minor amount of binder should be present. Suitable propellant compositions comprise:

Percent by Weight a. Binder 5 to 40 b. Catalyst 0.5 to 5 c. Carbon 0 to d. Stabilizer 0 to 5 e. Remainder of composition essentially ammonium nitrate Preferred compositions are as follows:

Percent by Weight a. Binder to 30 b. Catalyst 1.5 to 3.5 c. Carbon 1 to 5 d. Stabilizer 0.5 to 2.0

e. Remainder of composition essentially ammonium nitrate Other additives which may be added are present in an amount sufficient to provide effective improvement of the properties for which they are added.

In preparing the compositions of this invention, any procedure known to the art for the preparation of ammonium nitrate grains containing a thermoplastic binder may be used.

With respect to the properties of a solid propellant, the velocity at which a solid propellant is consumed during operation is called the burning rate. It is measured in a direction normal to the propellant surface and is usually expressed in inches per second. The burning rate may be expressed by the following relation, in which the influence of all performance parameters is small compared to the chamber pressure and the initial grain temperature:

The burning rate or velocity of propellant consumption r is usually given in inches per second; the chamber pressure p, in pounds per square inch; 0 and n are constants. The constant a varies with the initial propellant temperature, and thus the burning rate is a function of the temperature of the grain prior to combustion. The lower the value of n, the less is the potential for runaway burning under the influence of a pressure upset on a gas producing composition and the more constant is the burning rate of the propellant grain over a relatively wide range of chamber pressure. Thus, a sustained thrust rather than a detonation is obtained by smooth burning of the grain.

The temperature sensitivity for different solid propellants is usually expressed as the percentage change of thrust per unit of temperature change. Temperature changes effect the equilibrium pressure and the burning rate. The definitions of the temperature coefficients are given by Sutton, Rocket Propulsion Elements (2nd ed. 1958).

Here a is the temperature sensitivity coefficient of the equilibrium pressure at a particular value of K (K is the ratio of the area of the burning surface to the nozzle throat area), expressed in percent pressure change per degree temperature change. Mathematically it is defined as the partial derivative of the natural logarithm of the equilibrium chamber pressure P, with respect to temperature T. The other temperature sensitivity coefficient o, refers to the change in burning rate r of a solid propellant with respect to temperature T at a particular value of chamber pressure P,,. It is also known as the burning rate temperature coefficient, while 1n, is known as the temperature sensitivity of pressure.

For most propellant applications, as low a temperature coefficient as possible is desirable and even required for engineering design consideration. Lower pressure levels over a given temperature level allows a sizable weight savings for most missile applications.

DESCRIPTION OF PREFERRED EMBODIMENTS The following examples are given by way of illustration and should not be construed as limiting. In the following examples the component designated thermoplastic phenoxy resin was Bakelite Phenoxy Resin identified as Bakelite PKDA-8500 Resin.

EXAMPLE I A propellant formulation and the resultant strand ballistics are shown below.

The following example is given to illustrate the efiect of the novel stabilizer described above, on the stability of the following propellant formulation.

Component Wt. Thermoplastic Phenoxy Resin 13.34 Acetyl triethylcitrate 6.66 Tetranitroearbazole 4.00 Texas E carbon 3.00 2,4-toluenediamine 0.80 N-phenylmorpholine 0.20 Ammonium Oxalate 0.40 Sodium Barbiturate 1.50 Ammonium Nitrate 70.10

The above formulation was found to have an induction period of 190 hours in the standard gas evaluation test at 150C. Without the tetranitrocarbazole stabilizer an induction period of about 20 hours is about the maximum which may be expected.

EXAMPLE 111 Binder System Wt. Oxidizer Component Wt. Thermoplastic Phenoxy Resin 13.34 Acetyl tn'ethylcitrate 6.66 Tetranitrocarbazole 4.00 Carbon 3.00 Sodium Barbiturate 1.5 Toluene diamine 0.8 N-phenylmorpholine 0.2 Ammonium Oxalate 0.4 70.10 r 1000 psi/+70F. 0.085 '1 =0.83 up 0.15

EXAMPLE 1V Binder System Wt. Oxidizer Component Wt. Thermoplastic Phenoxy Resin 12.10 Acetyl triethylcitrate 8.72 Tetranitrocarbazole 3.49 Carbon 2.69 Sodium Barbiturate 1.5 Toluene diamine 0.8 N-phenylmorpholine 0.2 Ammonium Oxalate 0.4 70.10 0 r 1000 psi/+F. 0.069 n 0.70 op 0.13

EXAMPLE V Binder System Wt. Oxidizer 6 Component Wt. Thermoplastic Phenoxy Resin 11.70 Acetyl triethyleitrste 8.82

Tetranitrocarbazole 3 Carbon 2 Sodium Barbiturate Toluene diarnine N-phenylmorpholine Ammonium Oxalate 70.10

r 1000 psi/+70F. 0.068 1! =0.79 up 0.16

In the following examples, VIIX several binder systems were prepared and tested in a propellant composition to determine the strand ballistics. The binder systems and the oxidizer (ammonium nitrate), i.e., weight percentages of the components, and the type of plasticizer are given below.

EXAMPLE Vl Binder System Wt. Oxidizer Component Wt. Thermoplastic Phenoxy Resin 10.65 Dimethyl phthalate 8.00 71.00 r 1000 psi/+70F. 0.066 n 0.58

EXAMPLE VlI Binder System Wt. Oxidizer Component Wt. Thermoplastic Phenoxy Resin 10.77 Dibutyl phthalate 8.16 71.50 r 1000 psi/+70F. 0.055 n =0.52

EXAMPLE VIII Binder System Wt. Oxidizer Component Wt. Thermoplastic Phenoxy Resin 12.00 Dibutyl phthalate 8.00 71.50 r 1000 psi/+70F. 0.076 n =0.67 up 0.17

EXAMPLE 1X Binder System Wt. Oxidizer Component Wt. Thermoplastic Phenoxy Resin 12.70 Dibutyl phthalate 9.80 71.50

r 1000 psi/+70F. 0.054

1 claim:

1. A propellant composition comprising: a. a major amount of ammonium nitrate; and b. a thermoplastic resin binder having the following molecular structure:

([3113 Ill ire? wherein n is a number of such magnitude that the molecular weight of the resin is within the range of about 20,000 to about 40,000. 2. The composition of claim 1 wherein n is about 100.

3. The propellant composition of claim 1 containing a plasticizer selected from the group consisting of dimethyl phthalate and dibutyl phthalate.

4. The propellant composition of claim 1 containing a stabilizer of tetranitrocarbazole.

5. The propellant composition of claim 1 containing a catalyst selected from the group consisting of prussian blues and sodium barbiturate.

H050 UTHTED S' A'IES PATENT OFFICE (5/69) a r n n l CRETE JAKE Oi CORREUJK lQN Patent No. 3,720,553 Dated MARCH 3; 973

Inventor(s) LIONEL DE N It is certified that error appears in the above-identified paten't and that said Letters Patent are hereby corrected as shown below:

Column line 18: 13 should be in quotes Column line 19: The following letters should be in quotes Pc, a, n

Column line 20: EL should be in quotes Col umn l, line 23: n should be in quotes column line 4 and. column 7, line 3, "prussian blues" should read Prussian blues Signed and sealed this 20th day of November 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JRQ REI IE D. TEGTMEYER Attesting Officer Acting Commissioner of Patents P040 UNITED S ATES PATENT OFFIC S CERTM IQATE ()1? CQRREUMEUN Patent No. 3,720,553 Dated MARCH 3, 973

Inventor(s) LIONEL H NDERSON It is certified that error appears in the above-identified patent and that said Letters Patent are. hereby corrected as shown below:

r" I I "1 Column LP, line 18: 1; should be in quotes Column L, line 19: The following letters should be in quotes Pc, 8., n

Column line 20: a should be in quotes Column li ne23z n should be in quotes Column 3, line 4 and. column 7, line 3, "pruss'ian blues" should read Prussian b1ueso--' Signed an sealed this 20th day of November 1973.

(SEAL) Attest:

EDWARD M.FLETCHER;JRQ t RENE D. TEG'I'MEYER Attestlng offlcer Acting Commissioner of Patents 

1. A propellant composition comprising: a. a major amount of ammonium nitrate; and b. a thermoplastic resin binder having the following molecular structure:
 2. The composition of claim 1 wherein n is about
 100. 3. The propellant composition of claim 1 containing a plasticizer selected from the group consisting of dimethyl phthalate and dibutyl phthalate.
 4. The propellant composition of claim 1 containing a stabilizer of tetranitrocarbazole. 